(1) Field of the Invention
The present invention relates to an optical level control method suitable, for example, for use in a wavelength multiplexing optical transmission system.
(2) Description of the Related Art
In the recent years, high-speed data communications have encountered a striking increase in communication traffic, and for long-distance transmission, a wavelength multiplexing optical transmission network system (which will sometimes be referred to hereinafter as an “optical transmission system”) employing the wavelength division multiplexing (WDM) has been in increasing demand. In this optical transmission system, the distance between stations (offices), such as from a starting-point station to a repeater station, from a repeater station to a repeater station or from a repeater station to an end-point station, has frequently exceeded 600 km. For this reason, the optical transmission system has been put in operation by amplifying the optical output level considerably in view of the attenuation stemming from a loss in optical transmission lines.
In this long-distance optical transmission system, in a case in which a trouble or damage occurs in an optical transmission line such as an optical fiber (which will sometimes be referred to herein after simply as a “fiber”, a large optical output level can cause light leaking from the trouble occurrence place to undesirably affect workers involved in the correction of the trouble. A control system or method for eliminating this undesirable situation is called laser safety.
This laser safety is for the purpose of minimizing the influence on human body stemming from the fiber troubles, and as the approaches to realization of such laser safety, there have been known three types of methods of APR (Automatic Power Reduction), ALS (Automatic Laser Shutdown) and APSD (Automatic Power Shutdown). These approaches will be described hereinbelow with reference to FIGS. 25 to 28.
FIG. 25 is an illustration of a configuration of an optical transmission system. In FIG. 25, the optical transmission system, designated generally at reference numeral 200a, is for transmitting wavelength-multiplexed light, and is made up of WDM terminal stations 100a and 100d, repeater stations 100b and 100c, optical transmission lines 110, 112 and 114, and optical transmission lines 111, 113 and 115.
In the following description, let it be assumed that the term “up-direction” signifies a direction from the left side to the right side in illustrations while the term “down-direction” means a direction from the right side to the left side in the illustrations. In addition, let it be assumed that transmission of two kinds of light: main signal light (wavelength-multiplexed light, which will sometimes be referred to hereinafter as “WDM light” and OSC light Optical Supervisory Channel: sub-signal light) takes place in this optical transmission system 200a. 
The OSC light functions as a monitor control channel or pilot light. In addition, this OSC light is transmitted so as not to have influence on the transmission of the WDM light and is transmitted without passing through an optical amplifier (which will sometimes be referred to simply as an “amplifier”) thus resulting in extremely small output (optical power) as compared with the output level of the WDM light.
In this configuration, the WDM terminal station 100a wavelength-multiplexes light having a plurality of different wavelengths (λ1 to λn) for outputting the resultant WDM light to the optical transmission line 110, and further demultiplexes down-direction light outputted from the repeater station 100b for outputting the demultiplexed light having wavelengths (λ1 to λn).
Furthermore, the repeater station 100b repeats and amplifies WDM light from the optical transmission line 110 for sending out it to the optical transmission line 112, and further repeats and amplifies WDM light from the optical transmission line 113 for sending out it to the optical transmission line 111. The repeater station 100c conducts the repeating and amplification similar to those of the repeater station 100b. 
Still furthermore, the WDM terminal station 100d, as in the case of the WDM terminals station 100a, demultiplexes up-direction WDM light for outputting the demultiplexed light having wavelengths (λ1 to λn) and further wavelength-multiplexes light having a plurality of different wavelengths (λ1 to λn) for outputting the resultant WDM light to the optical transmission line 115.
Thus, on the upstream side of the WDM terminal station 100a, the WDM light obtained by wavelength-multiplexing light with the wavelengths (λ1 to λn) from the left side in FIG. 25 in a multiplexer (MUX) 20 is amplified in an amplifier (TA-1; Transmission Amplifier-1) 30, while OSC light is outputted from an OSC optical transmitting unit 10. In addition, the amplified WDM light and the OSC light are multiplexed with each other in a multiplexer 12 and outputted to the optical transmission line 110.
Meanwhile, on the downstream side of the WDM terminal station 100a, down-direction light (WDM light and OSC light) outputted from the repeater station 100b in FIG. 25 is branched in a branching unit 13 so that the branched OSC light is inputted to an OSC light receiving unit 15, while the other light, i.e., WDM light, is amplified in an amplifier 31 (RA-1; Receiving Amplifier-1). The amplified WDM light is demultiplexed in a demultiplexing unit (DMUX) 21 to output the respective wavelengths (λ1 to λn).
Referring to FIGS. 26 to 28, a description will first be given hereinbelow of a method of, when a damage or trouble occurs in an optical transmission line, detecting the place of the damage occurrence, then followed by a description about three types of methods for restoring the operations of optical transmission systems 200a to 200c after the retrieval of the damage.
In addition, a description will be given of the restoration (recovery) based on the three types of methods shown in FIGS. 26 to 28. In the optical transmission systems, if cutting-off of the optical out put or reduction of the optical output level is made through the use of laser safety other than the foregoing three types, then a restoration operation would be required to bring (restore) the optical output level back to the normal value after the recovery from the damage.
FIG. 26 is an illustration for explaining the APR method. In FIG. 26, let it be assumed that a fiber trouble has occurred at a place indicated by character A between the WDM terminal station 100a and the repeater station 100b. In a case in which this trouble results from fiber disconnection or break, since the repeater station 100b lies on the downstream side of the trouble occurrence place A, both of an amplifier (ILA1; In-Line Amplifier1) 32 and an OSC light receiving unit 11 in the repeater station 100b cannot receive the WDM light and the OSC light from the WDM terminal station 100a. 
In this case, the laser safety employing the APR method is conducted as follows. First, the amplifier 32 of the repeater station 100b receives WDM-LOL (Wavelength Division Multiplexing-Loss of Light) indicative of failure or impossibility of reception of the WDM light. Upon receipt of this WDM-LOL, the amplifier 32 outputs a control signal (APR Control, LOL-Detect) to lower the optical output level of an amplifier (ILA2; In-Line Amplifier2) 33 connected to the down-direction fiber. Thus, an amplifier 31 connected to the down-direction fiber in the WDM terminal station 100a receives a lowered input level of the down-direction WDM light.
In addition, the amplifier 31 detects the lowering of the downstream side WDM light level (WDM-ILD [Input Level Down or Inputted-light Level Down]) for dropping the WDM-signal light output level of an amplifier 30, connected to the up-direction fiber, on the basis of the WDM-ILD detection. In consequence, the level of the optical output leaking from the trouble occurrence place A also drops. Moreover, the level of the optical output leaking from the trouble occurrence place A is controllable through a series of operations based on the APR method.
On the other hand, two kinds of APR control restoration are taken after the retrieval of the fiber: one involves automatic restoration and the other involves manual restoration in which an operator forces that station to output WDM light from an amplifier 30.
In the case of the automatic restoration, the amplifier 30 tentatively outputs WDM light after the elapse of a predetermined period of time, and upon the retrieval of the fiber, an amplifier 32 of the second station detects WDM-LOL restoration and puts a control signal in an amplifier 33 so that the WDM light output level goes back to the normal level. Likewise, upon the WDM-ILD restoration by the amplifier 31 in the first station, a control signal comes in the amplifier 30 so that the WDM light output level goes back to the normal level. Furthermore, if the fiber trouble does not come to recovery yet, the tentative light outputted from the amplifier 30 does not arrive at the latter-stage amplifier 32 and the amplifier 30 continuously receives the APR signal from the amplifier 31 even after the elapse of a predetermined period of time; therefore, the amplifier 30 recognizes that the fiber trouble does not come to retrieval yet, thereby restraining the transmission optical level.
FIG. 27 is an illustration for explaining the ALS method. Let it be assumed that a fiber trouble occurs at a place indicated by A in FIG. 27 and the trouble at the place A is due to fiber break as in the case of the above-described APR method.
In this case, the laser safety based on the ALS method is as follows. First, an amplifier 32 connected to the up-direction fiber in a repeater station 110b standing on the downstream side of the trouble occurrence place A detects the failure of reception of the WDM light (WDM-LOL) and stops the WDM light output from the station to which it pertains. In addition, upon the WDM-LOL detection, the amplifier 32 in the repeater station 110b stops the WDM light output to the next repeater station 110c. 
Furthermore, an amplifier (ILA3; In-Line Amplifier3) 34 in the downstream side repeater station 110c, when detecting the failure of reception of WDM light from an optical transmission line 112 (WDM-LOL), also stop the up-direction WDM light output of the station it pertains to. In addition, when detecting the failure of the up-direction WDM light (WDM-LOL), an amplifier (RA-2; Receiving Amplifier-2) 36 in a WDM terminal station 110d standing at the last position in the down-direction stops the output of an amplifier (TA-2; Transmission Amplifier-2) 37 connected to an optical transmission line (down-direction fiber) 115 which is in opposed relation thereto.
Thus, the WDM-LOL is made to be detected by each of the stations and conveyed stepwise to the downstream side stations.
Moreover, also for the down-direction optical transmission line, as in the up-direction optical transmission line, each of the amplifier 35 in the repeater station 110c and the amplifier 33 in the repeater station 110b ceases the output of the down-direction WDM light from the state it pertains to in response to the WDM-LOL detection. In addition, the amplifier 31 of a WDM terminal station 110a lying at the last position of the down-direction optical transmission line ceases the output of the amplifier 30 on the optical transmission line.
The optical output leaking from the trouble occurrence place A is cut off in this way. In addition, the WDM light output leaking from the trouble occurrence place A is cut off through a series of operations according to the ALS method.
Furthermore, there are two kinds of ALS control restoration owing to the fiber retrieval: one is automatic restoration and the other is manual restoration in which an operator forces that station to output WDM light from the amplifier 30.
In the case of the automatic restoration, WDM light is tentatively outputted from the amplifier 30, and when the fiber is in a retrieved condition, the amplifier 32 in the repeater station 110b brings the WDM light back to the normal level on the basis of the WDM-LOL restoration and outputs it to the repeater station 110c. Moreover, the amplifier 36 in the WDM terminal station 110d issues a control signal to the amplifier 37 on the basis of the WDM-LOL restoration, thereby bringing the WDM-light level back to the normal level.
Likewise, each of the repeater station 110c, the repeater station 100b and the WDM terminal station 110a outputs WDM light in the regular condition so that the WDM light of the amplifier 30 in the WDM terminal station 110a returns to the normal level. In addition, after the elapse of a predetermined period of time, the WDM light output level from the amplifier 30 lowers, and if the fiber trouble does not reach retrieval yet, the ALS control restoration is not made in this case.
In this way, according to the automatic restoration, the WDM light output level lowered is once brought back to the normal value on trial after the elapse of a predetermined period of time. Moreover, when the trouble occurrence place reaches restoration, the amplifier 32 of the downstream side repeater station 110b receives WDM light so that the amplifier 32 releases the amplifier 33 of the repeater station 110b from the WDM light output level lowered condition.
Owing to this release, the amplifier 31 of the WDM terminal station 110a can receive the WDM light with the normal output level so that the optical output level of the amplifier 30 in the WDM terminal station 110a is brought back to the normal level.
FIG. 28 is an illustration for explaining the APSD method. Also in FIG. 28, a fiber trouble occurrence place is indicated at A, and that trouble stems from a fiber break.
In the APSD method, when an amplifier 32 existing in a repeater station 120b on the up-direction downstream side of the trouble occurrence place A and connected to an up-direction fiber detects WDM-LOL, the same amplifier 32 is made to stop the WDM light output of an amplifier 33 connected to the down-direction fiber.
Thus, unlike the ALS method, this APSD method stops the WDM light output only in a zone in which a fiber trouble has occurred, without spreading to the next station.
For achieving this, an amplifier 31 connected to an down-direction fiber in a WDM terminal station 120a detects WDM-LOL and stops an amplifier 30 connected to the up-direction fiber on the basis of the WDM-LOL detection, this cutting off the WDM light output leaking from the trouble occurrence place A.
The WDM light output leaking from the trouble occurrence place A can also be cut off through a series of operations according to the APSD method.
Incidentally, the APSD control restoration resulting from the fiber retrieval is the same as the APR control restoration.
As described above, both the automatic restoration and manual restoration are applicable to the above-mentioned three kinds of laser safety (APR, ALS and APSD method).
In addition, Japanese Patent Laid-Open (Kokai) No. HEI 9-46297 (which will be referred to hereinafter as “document”) discloses an optical output cutoff system designed to cut off output light for securing safety when disconnection or the like arises in an optical cable, which is suitable for long distance optical transmission with a repeater station.
However, in the case of APR method, when an output level of WDM light outputted from each amplifier is lowered or when an input level drop of WDM light is detected by a reception side amplifier, the WDM light level varies in accordance with the distance between stations, the number of wavelengths, or the like. For this reason, there is a problem in that it is difficult for an operator of the optical transmission systems 200a to 200c to set an appropriate level-down threshold value of WDM light.
In addition, although a transmission side amplifier lowers the level of WDM light to output the level-lowered WDM light, there is a possibility that the reception side fails to detect ILD, and in this case, the control according to the APR method becomes impossible.
Thus, in case in which the control based on the APR method does not take place normally due to fiber trouble, this creates a problem in that there is a probability that the APR control begins to execute while a fiber trouble doesn't occur.
Still additionally, an operator needs to visit a location of each station for measuring the WDM light output level to set a threshold value. However, in this case, there is a need to take into consideration an error stemming from the passage of time or the like, and the selection of a set value is extremely difficult.
Moreover, when a trouble has occurred in an optical transmission line, an operator may be exposed directly to light when the operation for the automatic restoration starts. Thus, difficulty is experienced in maintaining the laser safety. This comes about in common to the above-mentioned three types of methods. Still moreover, the optical transmission systems 200a to 200c are basically required to be put in operation without stopping the WDM light output. In other words, there is a need to avoid easy cease of the operations of the optical transmission systems 200a to 200c. 
Furthermore, in the conventional restoration, since WDM light is outputted compulsorily at a time interval, also at this time there is a probability that an operator, who tries to remove a fiber trouble, receives WDM light.
Still furthermore, in the case of the technique disclosed in the aforesaid cited reference, since the output of an amplifier stops even if a fiber connector or the like comes out in an intra-station unit, no fiber break occurs actually in an optical transmission line while still reacting to a pseudo-fiber-disconnection such as the coming-out of a connector in an intra-station unit, which can stop the WDM light transmission.